The present invention relates to an ion generation method and an ion irradiation method, and more particularly, to an ion generation method and an ion irradiation method that are effective for formation of a shallow diffusion layer. The present invention also relates to a filament suitable for an ion generation apparatus.
The ion implantation method (ion irradiation method) is widely employed as a method of forming pn junction by introducing with an impurity such as boron (B), phosphorous (P), arsenic (As), or the like into a semiconductor substrate. In the ion implantation method, an impurity can be introduced into a desired portion by accurately controlling the concentration and the depth of the impurity.
As high integration of the ULSI is promoted and the element size is reduced, the importance in the formation of a shallow pn junction is increased. The above-mentioned ion implantation method is one of the doping techniques employed widely in the semiconductor device manufacturing process, and is employed for the formation of the pn junction by combination with a heat process (annealing) that is conventionally executed after the ion implantation.
However, formation of a shallow pn junction using B as the p-type dopant includes many difficult points as explained below. First, B, which is a light element, brings about the remarkable channeling tale at the time of ion implantation. For this reason, lowering the energy of the accelerated voltage for shallow introduction of B causes lowering of the effective dose due to the influence from reflection, spattering and the like. Otherwise, ions cannot be extracted when the voltage is too low in accordance with the apparatus performance. Further, B has a large diffusivity in silicon, and therefore, it brings about, for example, the short channel effect of a pMOS transistor.
Thus, a process using a heavy element such as gallium (Ga) and indium (In), which is also a p-type dopant similarly to B, is noticed. In an ion irradiation apparatus (an ion generation apparatus) executing the above-mentioned ion implantation method (ion irradiation method), generally, gas is introduced into an arc chamber, or either a solid or a liquid is sublimated and its vapor is introduced into an arc chamber, to execute the ionization of In.
In the case of the In ion implantation, a chloride (InCl3) is known as a solid source. The present inventor found the following problem in the p-type dopant ion implantation methods using the solid of this kind. That is, in the case of a chloride, chlorine corrodes metal members of the apparatus.
Particularly, the etching reaction is strong in the arc chamber and the ion source chamber, and therefore, a filament for emitting thermoelectrons is corroded. For this reason, the ionization of In cannot be stably executed and a long-time work is extremely difficult. Specifically, the work can be executed in an only short time ranging one to four hours and the use of the chloride is not practical.
On the other hand, an organic gas source such as trimethyl indium (TMI), triethyl indium (TEI) is known as a gas source. TMI, which has a vapor pressure at a normal temperature, is ionized in a support gas such as Ar, for example, and an In ion beam is extracted therefrom.
However, the present inventor also found the following problem in the p-type dopant ion implantation method using an organic gas source of this kind. An organic gas drastically reacts with oxygen or water and is therefore very dangerous. Even in a vacuum apparatus the organic gas source is too dangerous as an ion source of the ion irradiation apparatus, when the suction of atmosphere caused by vacuum leak and the like, filling of TMI/TEI, the ion source maintenance after use of TMI/TEI, and the like are considered.
Incidentally, the ion source chamber serving as a heart of the ion implantation apparatus (ion irradiation apparatus) is largely classified into the Freeman type using a hot electrode, the Bernas type and the microwave type using magnetron.
Next, a method of extracting ions by taking advantage of the hot electrode with this apparatus will be explained simply. Ar gas and ion source gas or vapor are, for example, supplied through a gas inlet port of the ion source chamber (arc chamber) and thermoelectrons are emitted from the tungsten filament in the chamber. Further, the direction of movement of the emitted thermoelectrons is deflected, and therefore, the probability of collision of the Ar gas and the ion source gas or vapor introduced into the chamber to the thermoelectrons can be increased.
In these conventional ion source chambers, the source of the ions to be irradiated is generally introduced into the arc chamber as gas, or vapor obtained by sublimating the solid as mentioned above. Discharging is made to occur between the filament and an electrode which is opposite thereto, and the thermoelectrons emitted from the filament collide with the gas or vapor to make it ionized, and the ions to be irradiated are obtained and extracted from the chamber.
To achieve the above object, it is necessary to apply the high electric field to the filament and efficiently emit the thermoelectrons. For this reason, tungsten that is a refractory metal is generally used as the material of the filament. In the case of pure tungsten, however, if discharging continues, the temperature of the filament almost rises up to the melting point, and tungsten may be crystallized when the temperature drops after stop of discharging. In a next discharging, the temperature of the crystalline grain boundary rises up locally and therefore the filament is broken.
For this reason, adding a trace amount of metals such as Al, Si, K and the like to pure tungsten and raising the recrystallization temperature of tungsten to improve its strength at a high temperature has been conventionally executed.
Such a melting point raising technique of adding a trace amount of impurities to the tungsten filament is also used for a filament of a fluorescent lamp or the like. In the case of the filament for the ion generation apparatus chamber, however, the filament is in direct contact with the specific material gas that is introduced into the chamber as the ion source. Many specific material gases generally have corrosiveness and reactivity. Therefore, the environment of use of the filament for the ion generation apparatus chamber is more severe than that of the filament of the fluorescent lamp used generally in an inert gas atmosphere. For this reason, there is a problem that the lifetime of the filament for the ion generation apparatus chamber is short.
Further, when a partial pressure of desired gas is low, it is necessary to increase the filament current to obtain a desired ion current. However, even if a trace amount of metals such as Al, Si, K and the like are added to raise the recrystallization temperature of tungsten, inconvenience such as the breakage of the filament occurs as a consequence of the recrystallization of tungsten.
Even when the filament is not broken, the impurities segregate the recrytalline grain boundary, which prevents the thermoelectrons from being emitted from the filament.